1. Field of the Invention
The invention is concerned with techniques for measuring dimensions and possibly velocity of moving objects, particularly parallelepiped objects passing on a conveyor of a production line.
2. Description of the Related Art
Generally speaking, industry is tending more and more towards automating the manufacture of mass-produced articles. This automation, which makes it possible to achieve highly sustained rates of output, requires the provision of an increasingly accurate monitoring means, particularly for monitoring dimensions of articles during the course of production in order both to decide whether they are to be accepted or rejected and in order to keep an eye on progress on the line.
This monitoring requirement is encountered in the case of the manufacture of insulating panels obtained from mineral wool. After production of the material of which the final panel is constituted, the material passes in the form of a continuous layer on a conveyor while undergoing various processes which include a number of cuts, both longitudinal and transverse, in relation to its axis of travel, so that the end product is panels of usually parallelepiped form, prepared to the desired dimensions. Generally, the order of magnitude of the dimensions is between 1 and 4 meters for the finished length, between 1 and 40 cm for the finished thickness, after compression, and between 0.3 and 2.5 meters for the finished width.
Hitherto, in this field, dimensional measurements were carried out manually and intermittently at the end of the line.
It would be useful to be able to monitor the dimensions and even the velocity at one or a plurality of stages in the cutting program. First of all, this would guarantee that the end products are constant in quality by detecting those which depart from the standards, and by preserving the production "history" of each of the products for a posteriori inspection. In addition, this would make it possible, on a basis of this data, to envisage regulating production such as for example controlling a cutting operation.
Therefore, it is necessary to have continuous measurements without contact in order not to disturb the geometry of the object being measured, and to know the results in real time so that they can be stored and processed.
A visual monitoring device widely used hitherto is the linear camera, referred to as the CCD camera (Charge Coupled Device) comprising a lens and a sensor provided with a bar of juxtaposed photosensitive elements, i.e., pixels. Each pixel receives a quantity of luminous energy and converts it to an electrical charge directly proportional to the intensity of the light received and the exposure time. The desired measurement is obtained in analog form at a given image frequency as a function of the number of pixels "exposed" and the distance separating the camera from the object, which can be digitized for data processing.
As it happens, the aforementioned insulating panels, before or after any one of the cutting operations, follow a path on the conveyor which is normally constituted by at least one rectilinear segment, and which is confined by the conveyor itself. But these panels do not have any predetermined position which is absolutely fixed in relation to the conveyor insofar as they may be variably centered thereon and may be of different sizes. Therefore, these panels do not a fortiori have any predetermined position in relation to any fixed detection device installed in the proximity of the conveyor. Therefore, a conventional CCD camera cannot be used because in order to evaluate the dimensions of an object the position of the camera relative to the object must be exactly known.